(a) Field of the Invention
The present invention relates to a shift control method for an automatic transmission. More particularly, the present invention relates to a shift control method for an automatic transmission in which if an accelerator pedal is depressed by the driver during a lift-foot-up 3xe2x86x924 shift operation of a drive D range, shifting is immediately performed into a third speed rather than first into a fourth speed, thereby improving shift feel and acceleration characteristics.
(b) Description of the Related Art
In automatic transmissions used for vehicles, a shift control system performs control to realize automatic shifting into different speeds and shift ranges according to various factors including throttle opening, vehicle speed and load, and several engine and driving conditions detected through a plurality of sensors. That is, based on such factors, the shift control system controls a plurality of solenoid valves of a hydraulic control system such that hydraulic flow in the hydraulic control system is controlled, resulting in the shifting of the transmission into the various speeds and shift ranges.
In more detail, when the driver manipulates a shift lever to a particular shift range, a manual valve of the hydraulic control system undergoes port conversion as a result of the manual valve being indexed with the shift lever. By this operation, hydraulic pressure supplied from a hydraulic pump selectively engages a plurality of friction elements of a gearshift mechanism according to the duty control of the solenoid valves, thereby realizing shifting into the desired shift range.
In such an automatic transmission, shift quality is determined by how smoothly the friction elements are engaged and disengaged. Namely, when changing shift ranges, the timing between the engagement of a specific set of friction elements in relation to the disengagement of another specific set of friction elements determines the shift quality of the automatic transmission. Accordingly, there have been ongoing efforts to develop improved shift control methods that enhance shift quality by better controlling the timing of friction elements to engaged and disengaged states.
While driving in a drive D range of the automatic transmission described above, shifting is performed into at least one higher speed (when not already driving in the highest speed) when the driver releases the accelerator pedal. This is done to minimize shift shock. For example, if the accelerator pedal is released when driving in a third speed of the drive D range, shifting into a fourth speed is performed. If the accelerator pedal is again depressed, shifting is performed back into the third speed.
Before the completion of lift-foot-up 3xe2x86x924 shifting, however, if the accelerator pedal is again depressed by the driver, tip-in 4xe2x86x923 downshift control is realized. At this time, shifting is first completed into the fourth speed, after which shifting back into the third speed is performed. Such back-to-back shifting significantly reduces shift feel. That is, since the vehicle is first slowed by the shifting into the fourth speed then abruptly shifted back into the third speed, the vehicle tends to lurch forward. Also, shift responsiveness is negatively affected since shifting is not immediately performed into the third speed, thereby resulting in poor acceleration characteristics of the vehicle.
The present invention has been made in an effort to solve the above problems.
It is an object of the present invention to provide a shift control method for an automatic transmission in which if an accelerator pedal is depressed by the driver during a lift-foot-up 3xe2x86x924 shift operation of a drive D range, shifting is immediately performed into a third speed rather than first into a fourth speed, thereby improving shift feel and acceleration characteristics.
To achieve the above object, the present invention provides a shift control method for an automatic transmission comprising the steps of (a) determining a duty ratio of a first solenoid valve that controls friction elements that are engaged in a third speed and disengaged in a fourth speed, if it is determined in a transmission control unit that tip-in 4xe2x86x923 shifting is required as a result of the driver depressing the accelerator pedal during lift-foot-up 3xe2x86x924 shift control; (b) determining if a value resulting from subtracting turbine rpm of a present fourth speed shift state from turbine rpm of a third speed is greater than a first reference value if the duty ratio of the first solenoid valve is not a reference duty ratio; (c) controlling, if the condition of step (b) is satisfied, the first solenoid valve to an initial state using an initial reference duty ratio, controlling the first solenoid valve according to a predetermined duty, then determining if a time required for the duty control is greater than a first reference time; (d) determining if a present turbine rpm is greater than a sum of engine rpm and a second reference value 11 if the time required for the duty control in step (c) is greater than the first reference time; (e) outputting an initial duty of feedback control by the first solenoid valve then performing feedback control if the present turbine rpm is greater than a sum of engine rpm and the second reference value; (f) determining if an absolute value of turbine rpm minus engine rpm is less than a third reference value; (g) controlling, if the condition of step (f) is satisfied, the first solenoid valve to a predetermined duty, and determining if a second reference time has elapsed during the control of the first solenoid valve; and (h) controlling, if the condition of step (g) is satisfied, a second solenoid valve to a predetermined duty to control a friction element controlled by the second solenoid valve, thereby completing shifting.
According to a feature of the present invention, the third speed is maintained if the duty ratio is determined in step (a) to be at a predetermined level.
According to another feature of the present invention, if the condition of step (b) is not satisfied, 4xe2x86x923 shifting is performed after completion of shifting into the fourth speed.
According to yet another feature of the present invention, comprehensive control for engine optimization is performed simultaneously with step (c).
According to still yet another feature of the present invention, the comprehensive control is performed if it is determined that engine rpm minus turbine rpm is less than a predetermined value.
According to still yet another feature of the present invention, comprehensive control is completed simultaneously with the control of step (f).
According to still yet another feature of the present invention, it is determined in step (a) if the duty ratio of the first solenoid valve is 0%.
According to still yet another feature of the present invention, the reference duty ratio of step (b) is 0%.
According to still yet another feature of the present invention, the predetermined duty of step (g) is 0%.
According to still yet another feature of the present invention, the predetermined duty of step (h) is 0%.